70 
THE GEOLOGIST. 
limpid, and not in tlie least viscous. It may be boiled in a flask for a con- 
siderable time, and concentrated considerably without change. When 
heated in an open vessel, a ring of insoluble silica is apt to form around 
the margin of the liquid, and this may soon cause the whole to gelatinize. 
The solution is, as I said, durable in proportion to its purity. It is not 
easily preserved beyond a few days, unless considerably diluted. It 
becomes opalescent after a short time, and then the jelly separates ; and 
once separated it cannot be redissolved in water. When the jelly is 
formed suddenly it is always more or less opalescent. Formed slowl3^ it 
is a jelly, perfectly colourless and limpid, like rock-crystal. If touched 
slightly it gives rise to a vibratory tremor. It contracts, after a few 
days, even in a close vessel, and then pure water separates from it. It is 
a very curious fact that coagulation, or the separation of silica in the jelly- 
like state, is effected in the course of a few minutes by a solution con- 
taining one ten-thousandth part of any alkaline or earthy carbonate, but 
not by caustic ammonia or neutral or acid salts, nor by sulphuric, nitric, or 
acetic acid. Coagulation occurs in a short time after passing carbonic acid 
through the solution. A little carbonate of soda to it will make it so solid 
that it may be inverted without spilling. When it is suddenly made so, it 
is always opalescent, and not transparent. Dried by the air-pump iyi 
vacuo, at the ordinary temperature, it forms a beautiful, transparent, 
glassy mass of great lustre, no longer soluble in water, and which reminds 
one greatly of that beautiful variety of opal termed "hyalite." 
Ordinary silicate of soda is not at all \vhat is termed " colloidal ; " 
if silicate of soda were put into the hoop-vessel, and left there floating 
upon the water, it would pass through, to a certain extent, to the water, 
but there would be no separation of its constituents. When hydrochloric 
acid is added and the constituents eliminated, then this action is set up. 
This soluble form of silica unites with various organic matters, as, for 
example, with common gelatine, or with skin. In fact you may tan by 
means of silica, and produce leather containing as much as 70 per cent, of 
silica. 
We will now inquire whether there is any reason to suppose that a 
similar process may play any part in the operations of nature. The con- 
dition required is a soluble silicate dissolved in water, and the decomposi- 
tion of that silicate by some agent, such as hydrochloric acid. Does 
nature present us with any apparatus which can take the place of this 
so-called dialyser.P All that we want is the porous bed of some rock like 
sandstone, in some convenient position, and that sandstone will act exactly 
as the dialysing apparatus. 
In the separation of the silica every bubble of the fluoride of silicon 
as it passes up through the water becomes immediately decomposed, and 
a portion of the gas escapes, not being thoroughly in contact with the 
water everywhere, and produces a slight smoke. We have reason to 
believe that this solution may play an important part in the phenomena of 
nature ; for there is no difficulty in explaining how such a solution may be 
obtained as is requisite to exhibit the phenomena of dialysis ; and very 
probably in nature we may find conditions exactly suitable for dialysis. 
If this be the case, we shall be at no loss to understand how in many 
instances siliciflcation has occurred. We know that it has occurred, and 
to an enormous extent, in nature. The mineral termed " opal " is nothing 
more than amorphous silica containing a little water. The proportion oil 
\^ ater is not definite ; it is variable, the extremes being somewhere about 
3 per cent, and 13 per cent of water. Sometimes this opal exhibits mosli 
beautiful colours, and then it acquires the name of "precious opal.'' 
